The present invention relates to the growth of high quality silicon carbide crystals for specific purposes, and in particular relates to the production of high quality semi-insulating silicon carbide substrates that are useful in microwave devices.
The subject matter of the invention is related to the subject matter of commonly-assigned U.S. Pat. No. 6,218,680 and its progeny, U.S. Pat. Nos. 6,396,080 and 6,403,982; and also to Publication No. US 20020167010; the contents of each of which are incorporated entirely herein by reference. As set forth in these patents, high quality, semi-insulating silicon carbide substrates are technically desirable for electronic devices that operate in (or produce) microwave frequencies within the electromagnetic spectrum.
The term “microwaves” refers to electromagnetic energy in frequencies covering the range of about 0.1 gigahertz (GHz) to 1,000 GHz with corresponding wavelengths from about 300 centimeters to about 0.3 millimeters. Silicon carbide's very high electric breakdown field offers a primary advantage for microwave applications. This characteristic of silicon carbide enables devices such as metal semiconductor field effect transistors (MESFETs) to operate at drain voltages ten times higher than field effect transistors formed in gallium arsenide (GaAs).
Additionally, silicon carbide has the significant advantage of a thermal conductivity of 4.9 watts per degree Kelvin per centimeter (W/K-cm) which is 3.3 times higher than silicon and ten times higher than either gallium arsenide or sapphire. These properties give silicon carbide a high power density in terms of gate periphery measured in terms of watts per millimeter (W/mm) and also an extremely high power handling capability in terms of die area (W/mm2). This is particularly advantageous for high power, high frequency applications because die size becomes limited by wavelength. Accordingly, because of the excellent thermal and electronic properties of silicon carbide, at any given frequency, silicon carbide MESFETs should be capable of at least five times the power of devices made from gallium arsenide.
As recognized by those familiar with microwave devices, they often require high resistivity (“semi-insulating”) substrates because conductive substrates tend to cause significant undesirable coupling problems at microwave frequencies. As used herein, the terms “high resistivity” and “semi-insulating” can be considered synonymous for most purposes. In general, both terms describe a semiconductor material having a resistivity greater than about 1500 ohm-centimeters (Ω-cm), including high-resistivity crystals that can be referred to as “fully insulating.”
Typically, silicon carbide in its conventional form tends to be conductive. Thus, using silicon carbide as a resistive material requires some treatment of the material to obtain the resistive (semi-insulating) properties. In many cases, this treatment takes the form of adding states (“levels”) within the bandgap that trap electrons or holes (or both) within the bandgap and thus preclude semiconductive behavior. Such states are typically created by adding dopant atoms (elements) to the semiconductor, but can also be created by forming (or taking advantage of) crystal defects such as lattice vacancies and mispositioned atoms or complexes of these.
Microwave devices are particularly important for monolithic microwave integrated circuits (MMICs) which are widely used in communications devices such as pagers and cellular phones, and which generally require a high resistivity substrate. Accordingly, the following characteristics are desirable for microwave device substrates: A a high crystalline quality suitable for highly complex, high performance circuit elements, good thermal conductivity, good electrical isolation between devices and to the substrate, low resistive loss characteristics, low cross-talk characteristics, and large wafer diameter.
Other devices similarly may require or can benefit from high resistivity substrates. These include high electron mobility transistors (“HEMTs”), of which those formed in other wide bandgap semiconductors such as the Group III nitrides (e.g. GaN, AlGaN, InGaN) are particularly attractive. Therefore, higher-quality high resistivity substrates offer similar advantages for GaN HEMTs and related devices.
Given silicon carbide's wide bandgap (3.2 eV in 4H silicon carbide at 300K), such semi-insulating characteristics are theoretically possible. As one result, an appropriate high resistivity silicon carbide substrate should permit both power and passive devices to be placed on the same integrated circuit (“chip”) thus decreasing the size of the device while increasing its efficiency and performance. Silicon carbide also provides other favorable qualities, including the capacity to operate at high temperatures without physical, chemical, or electrical breakdown.
As those familiar with silicon carbide are aware, however, silicon carbide grown by most techniques is generally too conductive for these purposes. In particular, the nominal or unintentional nitrogen concentration in silicon carbide tends to be high enough in sublimation grown crystals (1−2×1017cm−3) to provide sufficient conductivity to prevent such silicon carbide from being used in microwave devices.
As noted above, semi-insulating, silicon carbide devices should have a substrate resistivity of at least 1500 ohm-centimeters (Ω-cm) in order to achieve RF passive behavior. Furthermore, resistivities of 5000 Ω-cm or better are needed to minimize device transmission line losses to an acceptable level of 0.1 dB/cm or less. For device isolation and to minimize backgating effects, the resistivity of semi-insulating silicon carbide should approach a range of 50,000 Ω-cm or higher.
Certain work in this area suggests that satisfactory semi-insulating SiC can be produced by introducing deep level dopants (i.e. those creating states at least 300 meV from both the conduction and valence band edges) such as vanadium into the crystal; e.g. U.S. Pat. No. 5,611,955.
Additionally, the presence of non-elemental deep levels or intrinsic point defects (i.e., features that affect the electronic characteristics of the crystal, but that do not result from elements) can affect the electronic properties of the crystal, including favorably contributing to raising the resistivity of the silicon carbide crystal. Such deep levels can result from, among others, carbon vacancies, silicon vacancies, carbon atoms in silicon positions, and silicon in carbon positions.
As set forth in U.S. Pat. No. 6,218,680 and its offspring, however, introducing additional elements into the crystal can raise other disadvantages. Thus, U.S. Pat. No. 6,218,680 shows how semi-insulating silicon carbide can be produced without vanadium domination by incorporating shallow dopants and intrinsic point defects to produce the desired characteristics, while reducing the amount of nitrogen present. The '680 patent also provides a relevant discussion of the disadvantages of various other techniques for attempting to produce semi-insulating (high-resistivity) silicon carbide.
In silicon carbide that is made semi-insulating using the techniques of the '680 and related patents, the net p-type doping is produced by boron (B) and is very uniformly distributed in the crystal.
The distribution of nitrogen, however, is quite non-homogeneous. In the sublimation growth of SiC (usually in the form of a circular boule growing in the axial direction) this uneven distribution exhibits itself as radial and axial variations in the resistivity in the crystal. In turn, these variations result in a relatively low yield (i.e., percentage of resulting substrates that are satisfactory) of the desired material. Stated differently, controlling the boron to a desired concentration is relatively easy, but controlling the nitrogen concentration to specific levels is relatively difficult. Accordingly, minimizing the nitrogen concentration is one simple method of minimizing or avoiding its variation. Nevertheless, overly-low levels of nitrogen may fail to provide the desired amount of shallow n-type dopants required to compensate the boron and (along with other factors) help provide the semi-insulating characteristics.